1. Field of the Invention
The present invention relates to a method for forming a plurality of optical thin-films for an optical device on a substrate at a high accuracy, and to an apparatus therefor.
2. Description of the Related Art
Optical communications using optical fibers have seen rapid development in recent years. Optical devices, such as various filters, used in optical communications are required to achieve high performance, i.e., highly accurate optical characteristics, to meet this development.
In order to satisfy such a requirement, it is essential to accurately control the thicknesses of layers deposited on a substrate during making a multilayer thin film for use in an optical device.
FIG. 20 shows a conventional deposition apparatus used in making optical thin films that require highly accurate thickness control. The deposition apparatus shown in FIG. 20 is an ion beam sputtering (IBS) apparatus.
The IBS apparatus is controlled by a deposition controller 103. In the IBS apparatus, a raw material for thin films is arranged on a target 207 of a main unit 100, is heated by the energy caused by bombardment of ion beams emitted from an ion gun 102, and is vaporized. Thin-films are made using molecules of plasmas of this raw material and thus exhibit a high density. Moreover, since deposition is performed in a high vacuum, the amount of contaminant is small, and high-quality thin films can be deposited at a high accuracy.
In this IBS apparatus, a multilayer thin-film having a designed thickness is deposited while controlling the thickness of each layer deposited on the substrate using a thickness monitor 101 for measuring the thickness of the layers deposited in the main unit 100. The thickness monitor 101 is either of a type that measures the thickness using a natural frequency of a crystal oscillator, i.e., a crystal thickness meter, hereinafter referred to as the xe2x80x9ccrystal monitorxe2x80x9d, or of a type that measures the transmittance or the reflectance of the thin-film formed on a substrate, i.e., a thickness sensor, hereinafter referred to as the xe2x80x9coptical monitorxe2x80x9d.
However, the crystal monitor and the optical monitor described above have the following drawbacks when they are used in making a multilayer thin-film requiring a high accuracy.
The crystal monitor has a high resolution in measuring changes in thickness d of the deposited layers and can accurately control the relative thickness of the deposited layer. However, a measurement error regarding the absolute thickness occurs as the thickness of a thin-film formed on the crystal oscillator changes. Thus, the detected thickness d is different from the actual thickness, which is a problem.
Moreover, since the crystal monitor indirectly measures the optical thickness, i.e., the mechanical thickness, without considering variation in the refractive index, the crystal monitor cannot respond to the changes in the optical thickness. This is because some layers have the same mechanical thickness but different refractive indices depending on the characteristics of the layers.
In contrast, the optical monitor can directly measure the optical thickness, i.e., dp=nxc2x7d, that takes into account changes in refractive index n. The optical monitor uses a measuring light having a wavelength xcex, a quarter of which is equal to the optical thickness dp of each layer, and processes this measuring light to determine changes in transmittance or the like over time, as shown in FIG. 21.
The deposition controller 103 detects that a thin-film having a required thickness dp is formed when the changes in the transmittance reach the extrema, such as at a time t1 or a time t2. The deposition controller 103 then stops the operation of the ion gun 102 and ends deposition of thin-films in the main unit 100.
However, when a film having a small optical thickness dp (=xcex/4) is deposited, the measuring light sometimes cannot be set at a suitable wavelength.
Referring now to FIG. 21, if a layer having a thickness corresponding to the thickness formed at the time t3 at a wavelength xcex3 is to be formed, i.e., the optical thickness that does not correspond to xcex3/4, the output from the optical monitor (the thickness monitor 101) does not show the extremum of transmittance at the time t3.
In contrast, extrema of the transmittance can be observed at the times t1, t2and t4, when the optical layers having thicknesses of xcex1/4, xcex2/4, and xcex4/4, respectively, are formed.
FIG. 21 shows the relationship between time and the transmittance data DT output from the optical monitor. The graph in FIG. 21 shows that an optical thin film having a thickness dp of xcex1/4 is formed at the time t1, an optical thin film having a thickness dp of xcex2/4 is formed at the time t2, and an optical thin film having a thickness dp of xcex4/4 is formed at the time t4.
Here, xcex1, xcex2,xcex3 and xcex4 each represent wavelengths of the measuring light.
Accordingly, in the conventional deposition apparatus, the designated optical thickness must be detected without using extrema, if the optical layer to be deposited has a thickness not suitable to be measured by an optical monitor, resulting in a larger variation, which is a problem.
An object of the present invention is to provide a method for forming optical thin films and an apparatus therefor that achieve highly accurate deposition control in which the crystal monitor and the optical monitor function to complement the drawbacks of each other.
In order to achieve this object, a first aspect of the present invention provides an apparatus for forming an optical thin film including: a thin-film forming unit for forming a thin film by depositing a substance on a surface of a substrate; an optical monitor for optically measuring the thickness of the thin film and outputting first thickness data; a crystal monitor for measuring the thickness of the thin film based on a crystal frequency and outputting second thickness data; and a thickness determining unit for controlling deposition by the thin-film forming unit based on one of the first thickness data and the second thickness data by switching the optical monitor and the crystal monitor. The thickness of the deposited film is normally measured with the optical monitor. However, when the thickness of the layer cannot be measured by the optical monitor because the thickness is excessively small or is not suited to be measured by the optical monitor, the crystal monitor is used instead of the optical monitor. Here, the thickness data of the crystal monitor is corrected by the coefficient calculated based on the thickness data of the optical monitor measured up to the point of switching from the optical monitor to the crystal monitor. In this manner, a multilayer thin film constituted from layers having various thicknesses can be formed.
Preferably, the thickness determining unit controls the deposition based on the first thickness data when the thickness of the thin film to be deposited is measurable with the optical monitor, and the thickness determining unit controls the deposition based on the second thickness data when the thickness of the thin film to be deposited is not measurable with the optical monitor. The thicknesses of the layers constituting the multilayer thin film are designed to form a suitable filter, and the designed thickness of each layer is input to the apparatus in advance. Accordingly, when a layer having a thickness not suitable to be measured with the optical monitor is formed, the crystal monitor is used from the beginning of the deposition instead of the optical monitor to control the thickness. Thus, in making multilayer thin film constituted from layers having various thicknesses, the ion gun can be stopped without delay, the thickness of each layer can be accurately controlled, and the deposited layers have designed thicknesses.
Preferably, the thickness determining unit corrects the second thickness data based on the first thickness data. The crystal monitor exhibit a high resolution in measuring the thickness; however, as a substance is deposited on a crystal oscillator, i.e., a thickness sensor, the second thickness data of the crystal monitor deviates from the actual value, i.e., the first thickness data, which is a problem. In order to overcome this problem, the second thickness data of the crystal monitor is corrected every time the deposition of one of the layers is completed so that the crystal monitor always has the thickness data as same as that of the optical monitor. In this manner, the thickness of the layer can be measured at a high accuracy. When a layer having a thickness which is not measurable with the optical monitor, the thickness of the layer can be accurately measured with the crystal monitor as with the optical monitor. Moreover, a multilayer thin film constituted from layers having various thicknesses can be formed.
Preferably, the thickness determining unit calculates the function of transmittance or reflectance from the first thickness data and estimates the time when the first thickness data output from the optical monitor coincides with a designed thickness data by multinomial regression over the calculated function. An example of the multinomial regression is a quadratic regression function. Since an extremum of change in transmittance can be detected before the extremum is actually reached, the endpoint of the deposition can be preliminarily set, and the ion gun can be stopped at the endpoint, i.e., the time when the extremum is reached, without delay. Thus, the layers having accurate designed thicknesses can be formed.
Preferably, the thickness determining unit calculates the function of transmittance or reflectance from the first thickness data and estimates the time when the first thickness data output from the optical monitor coincides with a designed thickness data according to changes in slope data obtained from regression calculation over the calculated function. Since an extremum of change in transmittance can be detected before the extremum is actually reached, the endpoint of the deposition can be preliminarily set, and the ion gun can be stopped at the endpoint, i.e., the time when the extremum is reached, without delay. Thus, the layers having accurate designed thicknesses can be formed.
Another aspect of the present invention provides a method for forming an optical thin film comprising: a thin-film forming step of forming a thin film by depositing a substance on a surface of a substrate; an optical monitoring step of optically measuring the thickness of the thin film so as to obtain first thickness data; a crystal monitoring step of measuring the thickness of the thin film based on a crystal frequency so as to obtain second thickness data; and a thickness determining step of controlling the deposition during the thin-film forming step based on one of the first thickness data and the second thickness data. The thickness of the deposited film is normally measured with the optical monitor. However, when the thickness of the layer cannot be measured by the optical monitor because the thickness is excessively small or is not suited to be measured by the optical monitor, the crystal monitor is used instead of the optical monitor. In this manner, a multilayer thin film constituted from layers having various thicknesses can be formed.
Preferably, the first thickness data is used when the thickness of the thin film to be deposited is measurable with an optical monitor, and the second thickness data is used when the thickness of the thin film to be deposited is not measurable with the optical monitor. The thicknesses of the layers constituting the multilayer thin film are designed to form a suitable filter, and the designed thickness of each layer is input to the apparatus in advance. Accordingly, when a layer having a thickness not suitable to be measured with the optical monitor is formed, the crystal monitor is used from the beginning of the deposition instead of the optical monitor to control the thickness. Thus, in making multilayer thin film constituted from layers having various thicknesses, the ion gun can be stopped without delay, the thickness of each layer can be accurately controlled, and the deposited layers have designed thicknesses.
A third aspect of the present invention provides an optical filter including a plurality of thin films made with the apparatus described above and by the method described above. In the optical filter, each layer accurately has a designed thickness even when the thicknesses of the layers differ irregularly from layer to layer. The optical filter has optimum characteristics as an optical thin film, such as a gain flattening filter (GFF), for accurately adjusting gains according to the frequency.
A fourth aspect of the present invention provides an apparatus for forming an optical thin film, including: thin-film forming unit for forming a thin film by depositing a substance on a surface of a substrate; an optical monitor for optically measuring the thickness of the thin film and outputting first thickness data, e.g., a transmittance data DT; a crystal monitor for measuring the thickness of the thin film based on a crystal frequency and outputting second thickness data, e.g., a frequency data DF; and thickness determining unit for controlling the deposition of the thin-film forming unit, wherein the thickness determining unit corrects the second thickness data by using the first thickness data and controls the deposition of the depositing unit based on the corrected second thickness data. According to this structure, the optical monitor is used to calculate the time when a thickness smaller than the designed value is reached, and then the crystal monitor having a high thickness measuring resolution is used to measure the thickness and determine the time when the deposited layer reaches the designed thickness. In this manner, the endpoint of deposition can be accurately determined. When a thin film having a designed thickness not suitable to be measured by the optical monitor is to be deposited, the crystal monitor having the thickness data corrected by the coefficient based on the thickness data of the optical monitor is used to measure the thickness from the beginning of the deposition. Accordingly, a multilayer thin film constituted from layers having thicknesses differing irregularly from layer to layer can be formed.
Preferably, the thickness determining unit has a first preset data of the first thickness data and a second preset data of the second thickness data used in determining the thickness, wherein the first present data is smaller than the second preset data. In other words, the first preset data to which the first thickness data is compared is smaller than the designed thickness for each layer of the filter, for example. After the first preset data has been reached, the deposition is controlled by the second data. That is, the endpoint of deposition is determined based on the measured values of the crystal monitor instead of the optical monitor. According to this structure, the measurement using the crystal monitor is performed after the extremum is estimated by approximation at a high accuracy so as to allow the crystal monitor to measure the thickness, i.e., to detect the endpoint of the deposition, at a high resolution even when the layers of the multilayer thin film have various thicknesses and various characteristics due to various refractive indices. As a result, the thickness of the deposited layer can be controlled at a high accuracy without delay, the ion gun can be stopped without delay, and a multilayer thin film constituted from layers having designed thicknesses can be obtained.
Preferably, the thickness determining unit calculates the function of transmittance or reflectance from the first thickness data and estimates the time when the first thickness data output from the optical monitor coincides with a designed thickness data by multinomial regression, such as a quadratic regression, over the calculated function. Since an extremum of change in transmittance can be detected before the extremum is actually reached, the endpoint of the deposition can be preliminarily set, and the ion gun can be stopped at the estimated endpoint, i.e., the time when the extremum is reached, without delay. Thus, the layers having accurate designed thicknesses can be formed.
Preferably, the thickness determining unit calculates the function of transmittance or reflectance from the first thickness data and estimates the time when the first thickness data output from the optical monitor coincides with a designed thickness data according to changes in slope data obtained from regression calculation over the calculated function. Since an extremum of change in transmittance can be detected before the extremum is actually reached, the endpoint of the deposition can be preliminarily set, and the ion gun can be stopped at the estimated endpoint, i.e., the time when the extremum is reached, without delay. Thus, the layers having accurate designed thicknesses can be formed.
Whereas the conventional method for forming an optical thin film measures the thickness of the deposited layer with only one of the optical monitor and the crystal monitor, a method of the present invention according to a fifth aspect includes: a thin-film forming step of depositing a material on a surface of a substrate to form a thin film; an optical monitoring step of optically measuring the thickness of the thin film so as to obtain first thickness data; a crystal monitoring step of measuring the thickness of the thin film based on a frequency so as to obtain second thickness data; and a thickness determining step of controlling the deposition during the thin-film forming step, in which the second thickness data is corrected by the first thickness data, and the deposition during the thin-film forming step is controlled based on the corrected second thickness data. According to this structure, the optical monitor is used to calculate the time when a thickness smaller than the designed value is reached, and then the crystal monitor having a high thickness measuring resolution is used to measure the thickness and determine the time when the deposited layer reaches the designed thickness. In this manner, the endpoint of deposition can be accurately determined. When a thin film having a designed thickness not suitable to be measured by the optical monitor is to be deposited, the crystal monitor having the thickness data corrected by the coefficient based on the thickness data of the optical monitor is used to measure the thickness from the beginning of the deposition. Accordingly, a multilayer thin film constituted from layers having thicknesses differing irregularly from layer to layer can be formed.
Preferably, during the thickness determining step above, a first preset data of the first thickness data for determining the thickness is smaller than a second preset data of the second thickness data. In other words, the first preset data to which the first thickness data is compared is smaller than the designed thickness for each layer of the filter, for example. After the first preset data has been reached, the deposition is controlled by the second data. That is, the endpoint of deposition is determined based on the measured values of the crystal monitor instead of the optical monitor. According to this structure, the measurement using the crystal monitor is performed after the extremum is estimated by approximation at a high accuracy so as to allow the crystal monitor to measure the thickness, i.e., to detect the endpoint of the deposition, at a high resolution even when the layers of the multilayer thin film have various thicknesses and various characteristics due to various refractive indices. As a result, the thickness of the deposited layer can be controlled at a high accuracy without delay, the ion gun can be stopped without delay, and a multilayer thin film constituted from layers having designed thicknesses can be obtained.
Another aspect of the present invention provides an optical filter including a plurality of thin films made with the apparatus described above and by the method described above. In the optical filter, each layer accurately has a designed thickness even when the thicknesses of the layers differ irregularly from layer to layer. The optical filter has optimum characteristics as an optical thin film, such as a gain flattening filters for accurately adjusting gains according to the frequency.